Re: [PATCHv2 04/28] mm: make remove_migration_ptes() beyond mm/migration.c

[Dave Hansen]

On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote
> We also shouldn't try to mlock() pte-mapped huge pages: pte-mapeed THP
> pages are never mlocked.

That's kinda subtle.  Can you explain more?

If we did the following:

	ptr = mmap(NULL, 512*PAGE_SIZE, ...);
	mlock(ptr, 512*PAGE_SIZE);
	fork();
	munmap(ptr + 100 * PAGE_SIZE, PAGE_SIZE);

I'd expect to get two processes, each mapping the same compound THP, one
with a PMD and the other with 511 ptes and one hole.  Is there something
different that goes on?